This invention relates to phase correction and control systems, and more particularly, to a frequency stabilizing circuit which includes a synchronized or locked oscillator, wherein a first frequency reference signal is compared with a variable frequency signal, the frequency stabilizing circuit generally known as a phase locked loop.
A phase locked loop is a device for producing an output signal which can track an input signal in frequency and exhibit a fixed phase relationship to the input signal. As the input signal changes in frequency, the output signal will likewise change in frequency in such a manner as to maintain the phase relationship between the input and output signals.
In the past, phase locked loops have been implemented using analog techniques. A basic analog phase locked loop consists of three fundamental parts: A phase detector, an amplifier/filter (or more simply, a filter) and a voltage controlled oscillator (VCO).
The phase detector is a device which detects the difference in phase between two input signals, and produces an output signal proportional to the amount of the phase difference. In a phase locked loop, the two inputs to the phase detector are the input signal to the phase locked loop, and the output signal of the voltage controlled oscillator, i.e., of the phase locked loop. The output signal from the phase detector is an analog signal, the magnitude which is representative of the amount of phase difference between the two input signals thereto, hereinafter referred to as an error signal.
The filter serves to remove any high-frequency components from the error signal produced by the phase detector, and provides a slowly varying output signal which is representative of the average error in phase between the output signal and the input signal.
The voltage controlled oscillator is an oscillator which generates an output signal having a frequency controlled by the slowly varying output signal of the filter. In a phase locked loop, the input signal to the voltage controlled oscillator is the output of the filter, and is an error signal representative of the difference in phase between the input signal and the output signal. Due to the feedback of the output signal to one input of the phase detector, the frequency of the voltage controlled oscillator is adjusted by the VCO input signal to maintain the fixed relationship between the input signal and the output signal.
While present day implementations of phase locked loops have employed varying amounts of digital techniques, analog techniques have always been present, to some extent, and most notably in the VCO section of the phase locked loop.
The present invention provides an implementation of the phase locked loop by using digital techniques entirely, including a digital VCO; namely, the frequency of the output signal of the digital VCO is related to the value of an n-bit digital input word (i.e., is a function of the value associated with the digital input word).